The present invention relates generally to magnetic resonance (MR) imaging. More particularly, the present invention relates to a pulse sequence for an MR imaging system in an inhomogeneous magnetic field.
The magnetic resonance (MR) phenomena involves providing a fairly strong static magnetic field (polarizing field B0, along the z-direction in a Cartesian coordinate system denoted as x, y, and z) throughout an image volume of the subject or area of interest (e.g., one or more anatomy of a patient being studied). Of the molecules comprising the subject or area of interest within the image volume, those nuclei having magnetic moments (i.e., those having an odd number of protons) attempt to align themselves with this static magnetic field. Such orientated nuclei, i.e., in a quiescent orientation, can be nutated (by controlled amounts) when a radio frequency (RF) pulse (excitation field B1), which is in the x-y plane and which is tuned to the Larmor frequency, is applied in its vicinity. The presence of the RF pulse causes a net aligned moment, Mz, of the orientated nuclei to be rotated or xe2x80x9cflippedxe2x80x9d at a certain flip angle into the x-y plane, to produce a net traverse magnetic moment, Mt. Once the RF pulse is terminated, the nutated or excited nuclei eventually return to their quiescent orientation and in the process emit certain RF signals, which can be detected and processed to form an MR image.
When utilizing these signals to produce MR images, linear magnetic field gradient pulses (gx, gy, and gz) along three mutually orthogonal axes are also applied in a predetermined sequence to spatially encode the RF MR signals, so as to produce a map or xe2x80x9cimagexe2x80x9d of the different nuclei populations (i.e., the various tissues) within a given image volume. Typically, the object to be imaged is scanned by a sequence of measurement cycles, in which the linear gradient pulses and the RF pulses are selectively superimposed on the static magnetic field in accordance with the particular localization method being used. The resulting set of received MR signals, also referred to as nuclear magnetic resonance (NMR) signals, are digitized and processed to reconstruct data representative of the volume of spatially encoded and nutated nuclei into an MR image, using one of many well-known reconstruction techniques.
Usually, extreme care is taken to ensure that the static or main magnetic field (B0) is uniform and temporally stable to minimize image distortion. Such a field can be created by a heavy permanent magnet or by a cryogenically cooled super conducting system. In either case, these requirements cause the magnet in an MR system to be very expensive to design, construct, and maintain. Inhomogeneity of the static magnetic field is held to an order of approximately one part in a million.
Whole body magnets are an example of magnets that can generate such a uniform field. Typically, whole body magnets are designed to provide a homogeneous magnetic field in an internal region within the magnet, e.g., in the air space between the magnetic pole plates for a C-type magnet. A patient or object to be imaged is positioned inside the region of the magnet (e.g., the air space) where the field will be homogeneous. The gradient and RF coils used to generate linear gradient pulses and RF pulses, respectively, are typically located proximate the main magnet and/or the patient, for example, within the inner circumference of the main magnet.
Presently, even whole body magnets, which are closed magnet structures, can only produce a homogeneous magnet field suitable for conventional MR imaging or spectroscopy at approximately its center air space region. By the end air space regions of the magnet and outside the magnet ends or edges, the magnetic field is of the type known as fringe fields and is sufficiently inhomogeneous to be unsuitable for conventional MR imaging. Thus, even with the expense of a closed magnet structure, only a limited region of such a magnet is usable without causing image distortions due to field inhomogeneity.
Another disadvantage of closed magnet structures lies in its closed magnet geometry. Typically, closed magnet structures provide limited access to the patient positioned therein for personnel, such as a surgeon, and are thus not suitable, for example, for use in surgical procedures that are guided by real-time MR imaging. The closed geometry also adds to patient discomfort, for example, for patients who may experience claustrophobia.
In contrast, open magnet structures, which are designed to provide access to the patient, overcome some of the problems of closed magnet structures discussed above. Open magnet structures also tend to be cheaper systems to design, construct, and maintain such that they are suitable for environments that do not require all the features associated with closed magnet structures. However, because open magnet structures do not provide one or more magnets, and correspondingly the magnetic fields generated therefrom, that surround or enclose the image volume location from as many sides as possible, the static magnetic fields generated at the image volume locations for open magnet structures are fringe fields that exhibit high field inhomogeneity. Such fringe fields include a much higher field inhomogeneity than those generated for closed magnet structures. Hence, it is very difficult to acquire quality images with such inhomogeneous fringe field using conventional pulse sequences, i.e., pulse sequences typically used in systems with minimal static field inhomogeneity at the image volume, such as, a spin echo pulse sequence.
Thus, there is a need for a pulse sequence configured to use an inhomogeneous static or main magnetic field to produce MR images suitable for clinical MR imaging. There is a further need for a pulse sequence configured to acquire data representative of an image volume in the presence of an inhomogeneous static or main magnetic field relatively quickly (e.g., more than one line of k-space data acquisition per excitation).
One exemplary embodiment relates to a method for magnetic resonance (MR) imaging using an inhomogeneous static magnetic field, the method. The method includes providing the inhomogeneous static magnetic field to an object of interest located within an imaging volume. The inhomogeneous static magnetic field includes a static gradient component oriented in a third direction. The method further includes providing a pulse sequence to the imaging volume. The pulse sequence includes a readout gradient pulse oriented in the third direction, a slice selection gradient pulse oriented in the third direction, a first phase-encoding gradient pulse oriented in a first direction, and a second phase-encoding gradient pulse oriented in a second direction. The method still further includes acquiring a plurality of echo signals emitted from the object of interest located within the imaging volume per excitation of the pulse sequence. The static gradient component comprises the readout gradient pulse and the slice selection gradient pulse.
Another exemplary embodiment relates to a magnetic resonance (MR) system configured to image using an inhomogeneous static magnetic field. The system includes a main magnet having a bore configured to provide the inhomogeneous static magnetic field at an imaging volume located outside the bore. The system further includes a control unit in communication with the main magnet and configured to provide a pulse sequence to acquire a plurality of lines of k-space data per excitation. The pulse sequence includes a readout gradient pulse oriented in a third direction, a slice selection gradient pulse oriented in the third direction, a first phase-encoding gradient pulse oriented in a first direction, and a second phase-encoding gradient pulse oriented in a second direction. The inhomogeneous static magnetic field comprises the readout and the slice selection gradient pulses.
Still another exemplary embodiment relates to a system for magnetic resonance (MR) imaging using an inhomogeneous static magnetic field. The system includes means for providing the inhomogeneous static magnetic field to an object of interest located within an imaging volume. The inhomogeneous static magnetic field includes a static gradient component oriented in a third direction. The system further includes means for providing a pulse sequence to the imaging volume. The pulse sequence includes a readout gradient pulse oriented in the third direction, a slice selection gradient pulse oriented in the third direction, a first phase-encoding gradient pulse oriented in a first direction, and a second phase-encoding gradient pulse oriented in a second direction. The system still further includes means for acquiring a plurality of echo signals emitted from the object of interest located within the imaging volume per excitation of the pulse sequence. The static gradient component comprises the readout gradient pulse and the slice selection gradient pulse.
Still another exemplary embodiment relates to an image generated by the steps of generating an inhomogeneous static magnetic field, the magnetic field including a static gradient component oriented in a third direction, and generating a pulse sequence including a readout gradient pulse oriented in the third direction and a slice selection gradient pulse oriented in the third direction. The image is further generated by the steps of acquiring a plurality of lines of k-space data per excitation of the pulse sequence, and reconstructing the plurality of lines of k-space data to generate the image. The static gradient component comprises the readout gradient pulse and the slice selection gradient pulse.
Yet still another exemplary embodiment relates to a pulse sequence for generating a magnetic resonance (MR) image. The pulse sequence includes a static gradient component of an inhomogeneous static magnetic field, the static gradient component oriented in a z-direction of a Cartesian coordinate system, and a first phase-encoding gradient pulse oriented in a x-direction. The pulse sequence further includes a second phase-encoding gradient pulse oriented in a y-direction, an excitation pulse, and a plurality of refocusing pulses. A plurality of echo signals are acquired per excitation of the pulse sequence.